One issue of importance for future scenarios concerns the treatment of black
carbon removal rates. Whereas most models projected changes for BC to be approximately
linear, the ULAQ and GISS models had somewhat different treatments. In the ULAQ
model, the reaction of O3 with BC leads to additional losses for
BC. This caused a change of -30% for BC in the A2 2100 scenario in the simulation
that included consideration of chemistry feedbacks (SC6) relative to the scenario
without changes in chemistry (SC3). The GISS model, on the other hand, did not
consider any heterogeneous loss, but did assume that the loss of BC associated
with wet scavenging depended on the interaction of BC with SO42-.
In this model, as SO2 emissions decrease, the lifetime of BC increased.
This model projects a 50% increase in BC lifetime (and thus in the concentrations
relative to emissions) in 2100 associated with the A2 scenario. Thus, the uncertainty
associated with the projection of BC concentrations is larger than that for
the other aerosol types.

In addition to the studies outlined above, one additional study was performed.
In this study, the A2 2100 emission scenario was used to simulate HNO3
concentrations in 2100 using the Harvard University model (see Chapter
4) and the present day anthropogenic NH3 emissions were scaled
by the increase in N2O emissions in 2100 from the draft SRES A2 scenario.
Then the model described by Adams et al. (1999) was used to estimate direct
forcing after condensation of the additional HNO3 and NH3
onto the calculated sulphate aerosol. The nitrate and ammonium burdens increased
by about a factor of 4.7 and a factor of 1.9, respectively, in the A2 2100 scenario.
The estimated forcing associated with anthropogenic SO42-,
nitrate, and ammonium aerosols increased from -1.78 Wm-2 to
-2.77 Wm-2. Thus, the control of sulphate aerosol in future
scenarios may not necessarily lead to decreases in forcing, if the levels of
ammonium nitrate in aerosol increase.

Two final considerations include the possible impact of chemistry and climate
changes on future concentrations. These were examined by the ULAQ model for
the A2 2100 scenario. Future concentration changes were small for the simulation
that included changes in chemistry only (scenario SC6). Climate feedbacks, however,
were significant. Aerosol concentrations changed by as much as -20% relative
to the model simulations that did not include climate change.

5.5.4 Linkage to Other Issues and Summary

As we have seen, anthropogenic aerosols may have a substantial effect on the
present day aerosol abundance, optical depth and thus forcing of climate. While
we have made substantial progress in defining the role of anthropogenic aerosols
on direct forcing, significant uncertainties remain, particularly with the role
of anthropogenic organic and black carbon aerosols in detemining this forcing.
Our ability to assess the indirect forcing by aerosols has a much larger uncertainty
associated with it. The largest estimates of negative forcing due to the warm-cloud
indirect effect may approach or exceed the positive forcing due to long-lived
greenhouse gases. On the other hand, there is sufficient uncertainty in the
calculation of indirect forcing to allow values that are substantially smaller
than the positive forcing by greenhouse gases. Other factors which have not
been assessed include possible anthropogenic perturbations to high level cirrus
clouds as well as to clouds in the reservoir between 0°C and -35°C.
As we have discussed, significant positive forcing is possible from aerosol-induced
increased formation of cirrus and/or from increased glaciation of clouds in
the region between 0°C and -35°C.

Concerns about aerosols derive from a number of other considerations. These
include visibility, toxic effects and human health, interactions of aerosols
with chemical processes in the troposphere and stratosphere, acid deposition,
and air pollution. Of these concerns, those associated with toxic effects and
visibility have led the industrialised countries to promulgate standards to
reduce the concentrations of aerosols in urban and also more pristine locations.
Also, concerns about the effects of acid rain have led to increased controls
over the emissions of SO2. As shown above, the SRES scenarios for
the future (Nakic´enovic´ et al., 2000) have all assumed that emissions
of SO2 will eventually decrease, and the A1T and B2 scenarios predict
that sulphur emissions start to decrease on a global average basis almost immediately.

Emissions of carbon aerosols in the future may not necessarily follow the
scenarios outlined for SO2 within the SRES scenarios. Furthermore,
older scenarios, developed by IPCC (1992), do not presume that SO2
emissions would necessarily decrease. We have evaluated a set of scenarios for
future aerosols that account for a range of possible future emissions. We noted
that one of the future effects that need to be evaluated includes the perturbation
to the cycles of natural aerosols. Another includes the interaction of chemical
cycles with aerosols and the prediction of how changes in other gas-phase emissions
may be perturbing aerosols. We must also consider how changes in aerosols may
perturb gas phase cycles. Our evaluation of these interactions has been necessarily
limited. Both the interaction of atmospheric chemical cycles with aerosols and
the perturbation to natural aerosol components through changing climate patterns
need to be better understood.